ui: add item selection indicator

Update the item backgrounds to respond to a selection.

app/src/main/java/org/oxycblt/auxio/image/ImageGroup.kt

@@ -110,6 +110,7 @@ constructor(context: Context, attrs: AttributeSet? = null, @AttrRes defStyleAttr
         super.onAttachedToWindow()
         invalidateAlpha()
         invalidatePlayingIndicator()
+        invalidateSelectionIndicator()
     }
 
     override fun setActivated(activated: Boolean) {

app/src/main/java/org/oxycblt/auxio/ui/recycler/AuxioRecyclerView.kt

@@ -21,6 +21,7 @@ import android.content.Context
 import android.graphics.Rect
 import android.util.AttributeSet
 import android.view.WindowInsets
+import android.widget.Checkable
 import androidx.annotation.AttrRes
 import androidx.core.view.updatePadding
 import androidx.recyclerview.widget.GridLayoutManager

app/src/main/res/color/sel_selectable_background.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<selector xmlns:android="http://schemas.android.com/apk/res/android">
+    <item android:color="?attr/colorSecondaryContainer" android:alpha="0.32" android:state_selected="true" />
+    <item android:color="?attr/colorSurface" />
+</selector>

app/src/main/res/drawable/ui_item_ripple.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <layer-list xmlns:android="http://schemas.android.com/apk/res/android">
-    <item android:drawable="?attr/colorSurface" />
+    <item android:drawable="@color/sel_selectable_background" />
     <item android:drawable="?attr/selectableItemBackground" />
 </layer-list>